Self-Terminated Artificial SEI Layer for Nickel-Rich Layered Cathode Material via Mixed Gas Chemical Vapor Deposition

Because of the higher specific capacity, nickel-rich layered cathode material has received much attention from the lithium-ion battery community. However, its cycle life is desired to improve further for practical applications, and unstable interface with electrolyte is one of the main capacity fading mechanisms. Here, we report a facile chemical vapor deposition process involving mixed gases of CO₂ and CH₄, which yields thin and conformal artificial solid-electrolyte-interphase (SEI) layer consisting of alkyl lithium carbonate (LiCO₃R) and lithium carbonate (Li₂CO₃) on nickel-rich active cathode powder. The coating layer protects from side reactions and improves the cycle life and efficiency significantly. Remarkably, the coating process is self-terminated after the thickness reaches ∼10 nm, leading to the coating layer to account for only 0.48 wt %, because of the growing binding energy between the gas mixture and the surface products. The self-termination is characterized by various analytical tools and is well-explained by density functional theory calculations. The current gas phase coating process should be applicable to other battery materials that suffer from continuous side reactions with electrolyte

CO₂ Enhanced Chemical Vapor Deposition Growth of Few-Layer Graphene over NiO_<i>x</i>

The use of mild oxidants in chemical vapor deposition (CVD) reactions has proven enormously useful. This was also true for the CVD growth of carbon nanotubes. As yet though, the use of mild oxidants in the CVD of graphene has remained unexplored. Here we explore the use of CO₂ as a mild oxidant during the growth of graphene over Ni with CH₄ as the feedstock. Both our experimental and theoretical findings provide in-depth insight into the growth mechanisms and point to the mild oxidants playing multiple roles. Mild oxidants lead to the formation of a suboxide in the Ni, which suppresses the bulk diffusion of C species suggesting a surface growth mechanism. Moreover, the formation of a suboxide leads to enhanced catalytic activity at the substrate surface, which allows reduced synthesis temperatures, even as low as 700 °C. Even at these low temperatures, the quality of the graphene is exceedingly high as indicated by a negligible D mode in the Raman spectra. These findings suggest the use of mild oxidants in the CVD fabrication as a whole could have a positive impact